staging: r8188eu: remove sta2sta_data_frame prototype

Remove the prototype for sta2sta_data_frame. Make the function static, it
is used only inside rtw_recv.c.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220707194914.63794-2-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>